The basic operation of the Coinola box pump remains the same,
regardless of whether it is the open variety used to create suction
only, or enclosed for the purpose of capturing the exhaust to blow
pipes.  In whatever case, the act of opening a bellows means pushing
against either atmospheric pressure alone, or atmospheric pressure plus
what, for all the pump knows, could be a simple variation of barometric
pressure.

This all constitutes inward pressure, while stiffeners are needed only
to prevent outward pressure from blowing out the bellows covering
material.  Of course outward pressure is created as the bellows is
closed -- but only enough to overcome the slight resistance of air
passing through the outer flap valve(s), a small fraction of that
needed to overflex the appropriate leather or rubberized cloth used
in this application.

Stiffeners _are,_ however, needed in the following situations where
significant outward pressure is generated within a bellows:

* Unenclosed pressure-only or combined pressure/suction pumps as
commonly found in orchestrions and band organs

* Early Duo-Art "steamboat" pumps having 84 notoriously inefficient
flap valves

* Most foot-pumped player pianos, because of the relatively large
size of the pumping bellows compared to the thickness of covering
material used.

Jeffrey Wood